Recently, much attention has been paid to a nonlinear optical material, which is to have a nonlinear optical property between its polarization and the electric field when a strong photoelectric field such as a laser ray has been imparted thereto.
This kind of material has generally been known as a nonlinear optical material and has been described in detail in, for example, Nonlinear Optical Properties of Organic and Polymeric Material, ACS SYMPOSIUM SERIES 233, David J. Williams (American Chemical Society, 1983); Organic Nonlinear Optical Material, M. Kato & H. Nakanishi (CMC Co., 1985); and Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 1 and Vol. 2, D. S. Chemla & J. Zyss (Academic Press Co.).
In order for the material to function effectively as a nonlinear optical material, the material must satisfy requirements in accordance with the order of the nonlinear optical effect for which it is to be utilized. For instance, for expression of a nonlinear optical effect of the second order, it is necessary that the polarization of the material to be induced by the electric field be asymmetric to the center. Heretofore, inorganic compounds such as lithium niobate (LN) and potassium dihydrogen phosphate (KDP) have been used as secondary nonlinear optical materials. However, these inorganic compounds could not always obtain a sufficient quality index, and much attention has come to be paid to .pi.-electron-conjugated organic compounds having an electron-donating group and an electron-attracting group, which are expected to obtain a great latent quality index. Under this situation, utilizing single crystals of organic compounds as a secondary nonlinear optical material has been actively investigated, with the result of finding p-nitroaniline derivatives, chalcone derivatives and phenylazole derivatives (for instance, described in T. Kobayashi, M. Umegaki, H. Nakanishi & N. Nakamura, New Organic Nonlinear Optical Material I, ibid. II, (CMC Co., 1991)).
However, it is extremely difficult to control the molecules of such organic compounds to be arranged asymmetrically to the center, and the difficulty is a great bar to the development of excellent secondary nonlinear optical materials comprising single crystals of organic compounds. As a method of effectively utilizing the excellent capacity of the organic compounds in the level of their molecules, an attempt has been made to utilize a so-called electric field-oriented high polymer material comprising a combination of the above-mentioned .pi.-electron-conjugated organic compound and a high polymer as a secondary nonlinear optical material, the material being oriented by imparting an electric field thereto. However, the electric field-oriented high polymer material has a drawback that the dipoles as oriented to one direction by the action of an electric field as imparted thereto are to be disordered (i.e., randomized) with the lapse of time, the disordering being called orientation relief, and a solution to this problem is desired (for instance, described in D. R. Ulrich, Mol. Cryst. Lig. Cyrst., 1990, Vol. 198, pp. 3-38).
As one means of overcoming the problem of orientation-relaxation, use of a high polymer having a high glass transition temperature (Tg) may be proposed. However, the field orientation of the high polymer is generally effected by heating the high polymer at a temperature higher than its Tg so that the mobility of the molecules of the high polymer may be kept elevated. The elevation of Tg of the high polymer to be oriented is to elevate the temperature for the electric field-orientation. It is known that the nonlinear optical constant of the field-oriented high polymer is in inverse proportion to the temperature during the electric field-orientation of the polymer. Therefore, use of a high polymer having a high Tg is contradictory to the result of obtaining an electric field-oriented high polymer having a large nonlinear optical constant.
Because of these reasons, a high polymer material which may be oriented in an electric field at a low temperature without orientation-relaxation of the field-oriented material is desired.